All-metal discharge tube

ABSTRACT

In a fast linear or toroidal zeta pinch device a discharge tube which fits precisely into the primary winding of said device, said tube being a series of anodized aluminum rings stacked upon each other and held together by a nonconducting epoxy layer which is coated with a slightly conducting layer, and then an outer structural layer.

United States Patent Phillips et al. [451 May 30, 1972 ALL-METALDISCHARGE TUBE [58] Field ofSearch ..328/233-238, 256

[72] inventors: James A. Phillips; Aldred E. Scholield' William L.Briscoe, all of 1.05 Alamos, N: Emmmehmh Mex. Attorney-Roland A.Anderson [73] Assignee: The United States of America as [57] ABSTRACTrepresented by the United States Atomic Energy Commission In a fastlinear or toroidal zeta pinch device a discharge tube which fitsprecisely into the primary winding of said device. [22] Filed: 9, 1971said tube being a series of anodized aluminum rings stacked [211 App]'No; 122,401 upon each other and held together by a nonconducting epoxylayer which is coated with a slightly conducting layer, and then anouter structural layer. [52] US. Cl..... ..328/233, 328/256 [51 Int. Cl...H05h 1/02 6 Claims, 2 Drawing Figures F 3II 5 I ff\\\\\\\\\\ 2 5PATENTEDMYBO I972 3. 6 67. 059

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INVENTOR. James A. Phillips BY A/dred E. .S'chofie/d William L. B! iscoeALL-METAL DISCHARGE TUBE BACKGROUND OF THE INVENTION The inventiondescribed herein was made in the course of, or under, a contract withthe U.S. Atomic Energy Commissron.

The present invention relates to a device which can be used as the tubecontaining the plasma in a zeta pinch device. In the past thesedischarge tubes were made of ceramic materials such as quartz oralumina. The advantages to be gained using a metal tube over the ceramicare as follows:

1. A metal conductor can absorb high energy pulses on the surfaceexposed to the plasma without melting due to its higher thermalconductivity.

2. The contamination of the plasma by a wall material released byevaporation and ion bombardment can be made less by a careful choice ofthe wall material. In particular, losses by bremsstrahlung radiation areapproximately Z (2 being the atomic number) and thus using low Z metalsthe radiation is reduced substantially. v 3. Fabrication of a dischargetube which closely fits inside the primary of the zeta pinch device ismade easier if machined out of metal. In the past it has been difficultto procure ceramic or quartz tubes which do not add prohibitively to theinductance.

A figure of merit (Qt has been determined for evaluating variousmaterials. It has been calculated that the temperature rise AT) of asurface for a flux Q of energy is given by t time,

C thermal conductivity,

k thermal capacity and p specific heat.

With AT determined by the melting point it is clear that metals aresuperior to insulators since metals have a higher thermal conductivityC. However, with metals the question arises how to reconcile theconflict between the high electrical conductivity of the wall and thenecessity for the axial electric field in a zeta pinch. For slow linearand toroidal devices such as Zeta and Tokamaks it is practicable to usecorrugated thin walls to keep the wall currents to tolerable proportionsby resistance. For the fast linear and toroidal zeta pinch devices withhigh field gradients (E, l kilovolt per centimeter), the wall currentswould be too large for a practical tube. There are two other ways bywhich wall currents can be reduced other than high resistance, e.g"., l)by subdivision of the wall, and (2) by high inductance. In the design ofthe fast linear or toroidal zeta pinch device at least four criteriamust be satisfied.

l. The material must have a high melting temperature to absorb energyflux from the plasma without vaporizing the surface. A conductor canabsorb higher energy pulses on the surface exposed to the plasma withoutmelting due to its higher thermal conductivity.

2. The design must allow high electric fields, approximately 1 kilovoltper centimeter, to be applied along the tube.

3. Dimensional tolerances are tight.

4. The tube must be able to withstand being pumped down to a lowpressure.

SUMMARY OF THE INVENTION A series of aluminum rings were stacked on topof each other. Aluminum was chosen as the material not because it hasthe most desirable thermal properties but the surfaces can be anodizedand thus prevent arcing between each of the rings. A total of 800 ringsformed a discharge tube length of approximately 9% inches. Tests haveshown that 300 volts could be applied between adjacent rings withoutbreakdown. The outer coaxial current return has also introduced adifficulty with this design. The capacity of each ring to ground,although much smaller than the inter-ring capacity, meant that theapplied high voltage is not uniformly divided down the ring train. Thisdifficulty can be avoided by grading the voltage uniformly down the ringassembly by surrounding the system with a relatively high resistance. Inthe present device we use a carbon-loaded epoxy havinga resistance ofabout ohms as the slightly conducting layer. The slightly conductinglayer is necessary to smooth the voltage gradient down the length of thedischarge tube. Without this layer most of the voltage of a shortelectrical pulse will be applied across only a few of the rings at thehigh voltage end of the tube. A catastrophic breakdown of the insulationbetween rings will occur. Meanwhile, the resistivity must be high enoughthat it does not completely short out the main discharge. An outer layerof fiberglass cloth impregnated with epoxy gives strength to theassembly while the innermost layer is an epoxy which cements the ringstogether.

The primary object of this invention is to provide a discharge tubeuseful in containing a hot plasma, said tube being stable in thepresence of the hot plasma and also being able to withstand extremeamperage and high voltage generated in a relatively short period in zetapinch device.

This object and other objects and advantages are set forth hereinafterin the following description of the preferred embodiment and drawings.

BRIEF DESCRIPTION or THE DRAWINGS FIG. 1 is a schematic cross-sectionalview of a linear discharge tube of this invention along centerline Cwhile FIG. 2 is a schematic view of the problem solved by thisinvention.

Referring to FIG. 1, there is shown the stacked rings 1 which areanodized on their flat surfaces 5 and are held together by anonconducting triple layer consisting of an epoxy 3 which cements therings together and holds the vacuum, a thin metallic but slightlyconducting coating 4, and an outer epoxy coating which is impregnated ina fiber glass cloth 2 which wraps the entire discharge tube and gives itstructural strength.

DESCRIPTION OF PREFERRED EMBODIMENT In the preferred embodiment shown inFIG. 1, the aluminum ring 1 is anodized on its flat surfaces 5. Therings are held together by any nonconducting compound such as an epoxywith appropriate hardeners added. The approximate dimensions of the ringare: height 0.010 inch; width, 0.30 inch; epoxy layer 3, 0.010 inchradial thick; slight conducting layer 4, 0.10 inch thick; and fiberglass impregnated with epoxy, about 0. 1 inch thick. (FIG. 1 is not toscale.)

Although the inventors have disclosed a triple layer surrounding theexterior of the tube, the said invention is not limited by theserestrictions since these layers only perform the function of (a) holdingthe rings together and vacuum within the tube, (b) a conducting layerthat smooths the voltage gradient down the length of the discharge tube,and (c) the fiber glass cloth wrap impregnated with epoxy givesstructural strength to the discharge tube.

Referring now to FIG. 2, there is shown the aluminum rings 1 with theiranodized surfaces 5 and positioned inside the primary 6 of the zetapinch device. The capacitance denoted as C is the capacitance of theinner rings generated when high voltage is applied through the primaryconductor 6. Capacitance denoted as C is the capacity of each ring toground and is much smaller in value than the inner ring capacity C Inthe prior an alumina and quartz were used as discharge tubes but thealumina became quite friable after several discharges which caused it tocrack or chip off and thus degrade the plasma stream which it surrounds.Quartz tended to vaporize and dilute the plasma. The alumina ring deviceof the present invention is very stable in a high voltage field andbecause of its anodized surfaces there is no arcing between the ringsand the epoxy wrap has a much lower inductance than either the aluminaor quartz.

7 C 7 nor: el v4 the 7'" 'vqophic br ..1 occur, .ul e ne main dis thecritical follows.

be h s een in talle connected to the ac n l of 10 at the 1 h a eakdownoflhe insli M en eanwhile, the resistivi enough hat it does not charge.cnteria for e de I. e insulation coating with vol s.

2. The tube of claim 1 in which its shape is linear.
 3. The tube ofclaim 1 in which its shape is toroidal.
 4. The inner layer of claim 1comprising an epoxy with appropriate hardeners added.
 5. Theintermediate layer of claim 1 comprising a carbon-filled epoxy with aresistance of about 150 ohms.
 6. The outer nonconducting layer of claim1 comprising a fiber glass cloth impregnated with an epoxy.